I highly recommend Fluxx. More information can be found at the webpage given in the source, but I do want to say it's an absolutely fantastic and hilarious game that also keeps a person very intellectually engaged. The way the game plays is to literally change the rules almost every turn, so everyone has to stay on their toes and be able to adapt to anything. That sounds chaotic, but a player who's able to plan ahead well using the current state of the board and what they have in their hand has a clear advantage over someone who isn't paying as much attention. Trust me, it's great.